This invention relates to a wireless communication system, and more particularly to a wireless communication system that includes a combination of an adaptive modulation scheme and a MIMO multiplexing scheme.
Recent cellular wireless communication system such as a cellular phone is required to transmit a variety of data from information consisting of a relatively small number of bits, such as voice/character mail, to information consisting of a large number of bits, such as mail with pictures attached, and video phone calls. To meet these requirements, in the cellular wireless communication system, a plurality of packet sizes and a plurality of modulation schemes corresponding to the amount of data (number of bits) to be transmitted are available. The system appropriately chooses a combination of the packet size and the modulation scheme according to the wireless propagation environment during the transmission, thereby improving the transmission efficiency. A system that adopts to a changing wireless propagation environment and selects the most appropriate modulation scheme to improve transmission efficiency is called an adaptive modulation scheme. This adaptive modulation technique is employed in the current third-generation cellular phone system standard.
For the next-generation cellular phone standard, to increase the transmission capacity within an allocated frequency band, a multiple input multiple output (MIMO) system that spatially multiplexes signals simultaneously on the same frequency by a plurality of antennas of a transmitter/receiver is being introduced. In a wireless communication system that employs the MIMO system, a transmitter simultaneously transmits different data streams from a plurality of transmitter antennas. A receiver receives the transmitted data using a plurality of receiver antennas, and separates the MIMO-multiplexed signal into the original data streams, and then demodulate them. The employment of the MIMO system increases the transmission capacity of data transmission/reception in an wireless communication system. Theoretically, effect of increasing the transmission capacity equals to doubling of a bandwidth when two antennas are provided, and tripling of a bandwidth when three antennas are provided.
In the adaptive modulation scheme, the transmitter side selects a packet size and a modulation scheme appropriate to the wireless propagation environment, encodes a data packet to be transmitted, and modulates the data packet by the selected scheme. The transmitter side sends the parameters used for the modulation together with the data. The parameters include, for example, a modulation scheme, an encoding rate, and a packet size. These parameters are called a modulation and coding scheme (MCS). The receiver side requires this MCS information for demodulating the data.
In the system that employs the adaptive modulation, data is transmitted through a data channel, and control information such as MCS information is transmitted through a control channel. The receiver side is incapable of demodulating the data channel unless it can determine which of the modulation schemes has modulated the transmitted data. Thus, the receiver side performs processing in order of first demodulating the control channel containing the control information, extracting the MCS information, and then demodulating the data channel.
For this reason, the receiver side has to secure a memory capacity large enough to temporarily store the data of the data channel before demodulation, until the demodulation of the control channel is completed. Further, it takes time to demodulate the control channel. The problem of delay in processing the data channel is further increased.
This problem has been pointed out in JP 2006-229635 A, which discloses a technique related to a baseband demodulation architecture suitable for the adaptive modulation scheme. However, in the system which employs the adaptive modulation scheme performs MIMO multiplexing, a MIMO stream separation process has to be carried out at a stage prior to demodulation of the data channel. Thus, the problems become greater.
Introduction of the MIMO system to the wireless communication system based on the adaptive modulation scheme necessitates the receiver to carry out MIMO stream separation in addition to normal data channel demodulation. Thus, when the MIMO system is employed for the adaptive modulation scheme, as compared with a case where only the adaptive modulation scheme is employed, processing steps of the data channel further increases, resulting in a much greater delay.
A large processing delay of the data channel causes a reduction in quality of real-time communication services such as voice (VoIP) or TV telephone. A cycle of closed-loop feedback control such as hybrid automatic repeat request (H-ARQ) is made longer, causing a reduction in transmission capacity.
This invention provides a wireless communication system for a MIMO adaptive modulation scheme, which can reduce a memory capacity for storing data before demodulation and suppress a processing delay of a data channel.